In general, molten iron is prepared in a converter such that oxygen is supplied into the molten iron to oxidize carbon (C), silicon (Si), manganese (Mn), etc., contained in the molten iron. The temperature of the molten iron increases by itself due to heat generated while the above-mentioned components are being oxidized. Here, in general, a scrap ratio at which operation is possible by using the self-generated heat of the molten iron is approximately 20%.
Accordingly, to increase an operation ratio of the scraps, a method in which a material capable of generating heat by reacting with oxygen (for example, silicon (Si) or carbon (C)) is added into the molten iron, has been used.
Alternatively, a method of using secondary combustion heat, which is generated when CO gas generated during decarburization refinement performed in the converter is reacted with oxygen once more to be changed into CO2, is also used. More specifically, a lance is positioned over molten iron contained in a converter, and an oxidative gas is then supplied into the molten iron to thereby generate heat through decarburization treatment of the molten iron.
In the decarburization treatment, a reaction in which carbon (C) in the molten iron and oxygen (O2) in the oxidative gas are reacted to generate CO is performed as described in the following Formula 1. Then, a reaction of the following Formula 2 (hereinafter, referred to as a secondary combustion) is performed, in which the CO generated by the above primary combustion and the oxygen in the oxidative gas are reacted again to generate CO2.C+½O2→CO  [Formula 1]CO+½O2→CO2  [Formula 2]
As such, the secondary combustion occurs such that oxygen blown from a lance and CO gas generated in a converter are mixed and reacted, and the blown oxygen generally has a speed lower than the speed of sound. Accordingly, to accelerate the secondary combustion, a method for blowing oxygen at a speed equal to or less than the speed of sound is required. Also, it is advantageous that a moving distance of an oxygen jet to the molten iron is made long so as to allow the reaction of oxygen to be maintained for a long time.
Accordingly, in related arts, the methods for reducing the flow speed of the oxygen jet to increase the secondary combustion, such as, 1) a method in which the height of a lance over the molten iron is raised, 2) a method in which the oxygen jet is distributed by using a porous lance nozzle, 3) a method in which the shape of the lance nozzle is changed. Here, the method 3) is the easiest method for increasing the secondary combustion, and thus methods for accelerating the second combustion by changing the nozzle in various shapes to make the speed of the oxygen jet to be the speed of sound or less are used.
For example, the secondary combustion was accelerated by using a lance which has an expanded portion with a non-circular section so that the flow speed of the gas becomes the speed of sound or more before a throat provided in the lance, and becomes the speed of sound or less in a lower portion of the throat.
Alternatively, a plurality of separate gas supply holes are disposed at sidewalls of gas discharge holes of a general porous oxygen nozzle formed in the lance. Here, a method of reducing a flow speed of a gas jet by supplying gas to the gas supply holes to provide a swirl flow to the oxygen jet is used.
Alternatively, a lance, which changes a spraying direction of the sprayed gas without a decrease in a flow speed of oxidative gas sprayed from the lance and a changing in an inclined angle of a nozzle, is proposed. That is, proposed is a method, in which at least one nozzle in a lance provided with at least two gas spraying nozzles periodically sprays the gas along a direction crossing a straight line connecting a center point of the gas spray nozzle and a center point of a lateral cross-section of the lance. It is asserted that this method can improve the secondary combustion, and allow base metal attached to the wall of a converter to be uniformly melted.
Alternatively, a hole through which a control gas may be supplied is formed at a side surface of an expanded portion of a supersonic nozzle, and the control gas is blown therethrough. Accordingly, methods for changing the direction of the blown oxidative gas and reducing the speed are also used.
Also, a lance, in which expansion nozzles are disposed at a front end of the lance to face each other at positions of point symmetry with respect to the lance center, and a straight nozzle is disposed at other positions of a same concentric circle, is proposed to uniformly melt the base metal attached to the inner wall of a converter.
However, related art methods are used to generate a secondary combustion such that oxygen at room temperature is supplied into a converter and is reacted with high-temperature CO generated during decarburization. Accordingly, a temperature higher than a predetermined level is required for the combustion of CO. The second combustion does not occur until the oxygen at room temperature is mixed with CO and the temperature is raised. Thus, in general, in order that the blown oxygen may be slowly reacted with CO gas from a surface thereof, a secondary combustion may then occur, and the entire oxygen jet to the inside thereof may be used for the secondary combustion, the oxygen jet should move a substantial distance.